On Thursday the Glenrothes by-election will take place. It was always going to be an important poll, since it would show whether the surge towards the Scottish National Party was still rising. Now it has become even more significant. If it can hold the seat, Labour will be looking very closely at the possibility of an early election. Michael Portillo analysed this option in his column in The Sunday Times yesterday; he represents a widely held opinion. There are signs of preparations in the Labour Party itself.

The argument runs like this. Labour has little hope of winning an election in late 2009 or 2010. The world economy is unlikely to recover in the next 18 months, and might get considerably worse. The Government may get the blame for rising unemployment, negative equity in housing and increasing misery. The public have been impressed by Gordon Brown's more decisive handling of the banking crisis. The recession, as it develops, may wipe out that favourable impression.

For Labour, the optimistic scenario starts, and may well finish, at Glenrothes. If Labour holds its seat on Thursday, even narrowly, the option of an early election would begin to become a real one. The SNP under Alex Salmond would no longer look like an irresistible force. Labour could expect to hold many, perhaps most, of its Scottish seats; it has to do so if it is to have any chance of an overall majority at a general election.

During the 1997 general election, I spent some time in the Glenrothes constituency. Our son, Jacob, had just come back from Hong Kong, and was that rare thing, an English Conservative candidate for a Scottish seat. He had no chance of winning, but his team canvassed some 20,000 houses during his campaign.

I'm afraid that Glenrothes, a new town built on the now defunct Fife coalfield, is a failure of Labour social engineering. In 1956 and 1959, I had myself fought Chester-le-Street, now famous for its cricket field. The Labour Establishment in Durham had done a much better job of reconstruction than it has in central Fife. Judged by any social criteria, Labour deserves to lose Glenrothes.

However, if it does hold the seat some Conservatives fear that the opinion polls will move further in Labour's favour. At present the polls are not favourable enough for Mr Brown to hold an election. A YouGov poll in last Thursday's Daily Telegraph showed 42 per cent for the Conservatives, 33 per cent for Labour and 15 per cent for the Liberal Democrats. This represents a drop in Conservative support of 3 per cent, and a Labour gain of 2 since October 4. At a general election, the Rallings and Thrasher formula suggests that, on these figures, the Conservatives would have 331 seats to Labour's 260; the overall Conservative majority would only be 12, but the majority over Labour would be 71 - perhaps enough to last a full Parliament.

Those who expect an early election think that a victory in Glenrothes might be followed by a rally of support for Labour. They talk of the possibility that the Conservative lead would be cut to 3 per cent or some such low figure. They then argue that Labour could hope to gain further support in the campaign itself.

There certainly are signs that Mr Brown is again considering the possibility of an election. In 2007 he was tempted to go to the country and cash in on his honeymoon popularity as Prime Minister. The autumn election that never was may have been his great missed opportunity. Even then he must have felt very doubtful about his prospects if he waited for the latest possible election date, in May or June 2010. Such doubts would not have been removed by the recession. The Prime Minister would now like to call an early general election if he thought he could win it. That can never be a certainty.

The return of Labour's two great election experts, Lord Mandelson and Alastair Campbell, has been taken as a sign that Mr Brown is considering the option. David Blunkett has also been seen visiting Downing Street. This does look like the reappearance of the old new Labour general election team.

The Conservatives have good reason to be afraid of them. They may not be particularly sympathetic characters, but they are good at their job, and their job is political propaganda. One should never underrate Tony Blair's personal contribution to the election victories of 1997, 2001 and 2005, but these men are the A-team of electioneers who, between them, have won Labour 11 years in office.

I feel sure that Mr Brown will think about an election, but I doubt if it will happen. There are too many weaknesses in the argument. Labour is not ahead in the polls, but behind. The recession is not getting better, but worse. The Labour Party lacks both enthusiasm and membership. The SNP is formidable, whether or not it wins Glenrothes. Even the timing is bad. It would be possible to hold an election in December, but that is cold midwinter. Each week from now on, unemployment will rise, house prices will fall and the voters will become even more conscious of the recession.

We know that Mr Brown is a cautious man. Before calling an election he would like to be ahead by 5 per cent or so, and to have consolidated that lead over the previous six months. He would like to have a big theme for an election and to have gained more credit for his response to the recession.

Then he could have reduced his risk.

As things stand, he is too far behind in the polls. It is not clear what convincing theme he could offer the electorate. It is almost always the incumbent party that is blamed for a recession, as the Republicans were in the US in 1932 and seem to be again in 2008. The strongest reason for Labour to call an early election is that they expect to lose one later on.

“Vote Labour now, because you won't want to in 2010” would not make an inspirational slogan.

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